$^{19}$F NMR and defect spins in vacuum-annealed LaO$_{0.5}$F$_{0.5}$BiS$_2$

TL;DR

This study uses $^{19}$F NMR and magnetization measurements to investigate defect spins and magnetic behavior in vacuum-annealed LaO$_{0.5}$F$_{0.5}$BiS$_2$, revealing local moments and conduction-band fluctuation effects without magnetic order.

Contribution

It provides the first detailed analysis of defect-induced local moments and their magnetic interactions in LaO$_{0.5}$F$_{0.5}$BiS$_2$ using NMR and magnetization data.

Findings

Local moments are present at ~1000 ppm concentration.

No magnetic order observed down to 2 K.

Defect spins interact via dipolar interactions and conduction-band fluctuations.

Abstract

We report results of magnetization and $^{19}$F NMR measurements in the normal state of as-grown LaO$_{0.5}$F$_{0.5}$BiS$_2$. The magnetization is dominated by a temperature-independent diamagnetic component and a field- and temperature-dependent paramagnetic contribution $M_\mu(H,T)$ from a $\sim$1000~ppm concentration of local moments, an order of magnitude higher than can be accounted for by measured rare-earth impurity concentrations. $M_\mu(H,T)$ can be fit by the Brillouin function $B_J(x)$ or, perhaps more realistically, a two-level $\tanh(x)$ model for magnetic Bi $6p$ ions in defect crystal fields. Both fits require a phenomenological Curie-Weiss argument $x = \mu_\mathrm{eff}H/(T + T_W)$, $T_W \approx 1.7$ K. There is no evidence for magnetic order down to 2 K, and the origin of $T_W$ is not clear. $^{19}$F frequency shifts, linewidths, and spin-lattice relaxation rates are consistent with purely dipolar $^{19}$F/defect-spin interactions. The defect-spin correlation time $\tau_c(T)$ obtained from $^{19}$F spin-lattice relaxation rates obeys the Korringa relation $\tau_cT = \text{const.}$, indicating the relaxation is dominated by conduction-band fluctuations.